This invention relates generally to the field of antenna structures for radio communications equipment and more particularly to a shortened decoupled wideband sleeve dipole antenna flexibly realized for use in duplex portable radio applications.
It is well established in the field of antennas that a quarter wavelength monopole mounted perpendicularly to a conducting surface provides an antenna having good radiation characteristics, desirable drive point impedance, and relatively simple construction. Such antennas have been disclosed in U.S. Pat. Nos. 3,611,402 and 3,624,662 assigned to the assignee of the present invention. The necessity of a conducting surface makes monopole antennas an attractive choice for mobile applications where the metallic body of a vehicle serves particularly well as a ground plane conducting surface. Monopoles have also been employed as antennas for hand-held portable transceivers, such as referenced in U.S. Pat. No. 4,121,218 assigned to the assignee of the present invention, but the detuning and absorbtive effects of the user's body have indicated that monopole antennas are not particularly suited for portable applications.
Additionally, if the transceiver is to be operated in a duplex mode--that is, the transmitter and receiver operating simultaneously--the relatively high power radio frequency currents present in the metallic chassis of the transceiver when used with a monopole antenna tend to disrupt the operation of the receiver. One solution to this problem found in duplex operation is disclosed in U.S. Pat. No. 4,138,681 assigned to the assignee of the present invention, in which currents in the chassis of the portable are reduced by employing antenna radiating elements decoupled from the portable chassis.
A solution to the ground plane requirement of the monopole antenna is the use of a dipole antenna. This solution is also quite well known and commonly employed at VHF and UHF frequencies. One such antenna structure for portable transceiver equipment was disclosed in U.S. Pat. No. 4,205,319 assigned to the assignee of the present invention. Half-wave dipoles, however, are physically large when compared to the relatively small portable transceiver. Such large dipoles are both aesthetically displeasing and cumbersome to the user of miniature portable transceivers.
Reduction of the physical size of portable transceiver antennas has generally been achieved by employing helically wound radiators for one element of the dipole (see U.S. Pat. Nos. 3,720,874 and 4,504,834 assigned to the assignee of the present invention) or for both elements of the dipole (see U.S. Pat. No. 4,442,438 assigned to the assignee of the present invention). Physical size reduction, however, reduces the operating bandwidth of the antenna (generally recognized as the frequencies at which the return loss is greater than -10 dB) because of changes in the input impedance.
Since a duplex portable transceiver typically requires at least one frequency for radio frequency signal transmission and at least one different frequency for radio frequency signal reception, the antenna should include both frequencies within its operating bandwidth. The requirement is further complicated if the portable transceiver is to be used in a cellular radiotelephone system where a multitude of frequencies in one band are potentially useable for transmission and a multitude of frequencies in another band are potentially useable for reception. The antenna for a cellular portable radiotelephone, then, must either have a very broad bandwidth or have two bands of operating bandwidth to function properly with the portable. Broadband or dual bandwidth antennas have been realized in several recent inventions (see U.S. Pat. No. 4,442,438,4,494,122; and 4,571,595, each assigned to the assignee of the present invention). Generally, these antennas are physically longer and stiffer than desirable in a portable cellular radiotelephone and leave a need which can be fulfilled by the present invention.